Effect of non-active area on the performance of subgasketed MEAs in PEMFC

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 7 4 0 0 e7 4 0 6

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Effect of non-active area on the performance of subgasketed MEAs in PEMFC Xinsheng Zhao, Yongzhu Fu, Wei Li, Arumugam Manthiram* Materials Science and Engineering Program and Texas Material Institute, University of Texas at Austin, Austin, TX 78712, United States

article info

abstract

Article history:

Subgaskets are usually applied to a catalyst-coated membrane (CCM) for the edge-

Received 16 February 2013

protection of the electrolyte membrane and easy handling. They cover the peripheral re-

Received in revised form

gion (non-active area) of CCM and have a defined window (active area) for accommodating

28 March 2013

the electrode. In this study, three subgasketed CCMs with different configurations were

Accepted 29 March 2013

designed with a laboratory-scale 5 cm2 fuel cell and the effects of the components un-

Available online 3 May 2013

derneath the subgaskets on the electrochemical properties of CCMs and cell performance were investigated by several electrochemical techniques. The results reveal that part of the

Keywords:

catalyst layer under the subgaskets is activated for reaction area, leading to slightly higher

Catalyst-coated membrane

electrochemical surface area (ESA), higher H2 crossover, and smaller shorting resistance.

Subgasket

The non-active region of subgasketed CCM has little impact on proton resistance in the

Hydrogen crossover

catalyst layer, oxygen reduction reaction (ORR) kinetics, and limiting current, but has

Single cell performance

adverse effects on cell performance in the low current region under dry conditions due to increased hydrogen crossover. The findings could provide guidelines for subgasket design and application in laboratory-scale fuel cells. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

A membrane-electrode assembly (MEA) typically consists of proton exchange membrane (PEM), gas diffusion layers (GDL), and catalyst layer (CL). The catalyst ink can be deposited on PEM or GDL, which are known, respectively, as catalyst-coated membrane (CCM) and catalyst-coated gas diffusion layer (CC-GDL). For the purpose of sealing, the electrolyte membrane is generally larger than the electrode and its edges protrude outward between the two electrodes. The protective films made from the inert and dimensionally stable materials (e.g. PET (polyethylene terephthalate), PEN (polyethylene naphthalate) or Kapton(polyimide)) are attached to the edges of the electrolyte membrane to enhance the mechanical strength and stiffness of the edge.

The protective films are termed as the “subgakets” in fuel cell and applied on both sides of the MEA [1e4]. As illustrated in Fig. 1, the subgaskets cover the peripheral region (non-active area) of the catalyst layer and have central openings (defined as electrode window) accommodating the catalyst layers that are configured to be the same size as the electrode windows. The electrolyte membrane is, therefore, interposed between two pieces of subgaskets and does not appreciably extend beyond the outer edges of the subgaskets. The subgasket plays several roles: (1) edge protection of the membrane to avoid fast mechanical failure and chemical degradation; (2) positioning the anode and cathode to eliminate the misalignment of electrodes; (3) blocking external contamination and preventing lateral leakage of the reactant gases; and (4) easy handling during fuel cell stack assembly. For

* Corresponding author. Tel.: þ1 512 471 1791; fax: þ1 512 471 7681. E-mail addresses: [email protected], [email protected] (A. Manthiram). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.03.160

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 7 4 0 0 e7 4 0 6

Gas diffusion layer

Subgasket Adhesive

7401

Catalyst layer

Proton exchange membrane

Fig. 1 e Cross-sectional diagram of MEA with subgaskets.

example, several groups [5e9] have used subgasket to prevent electrolyte membrane from excessive pressure and stress at edges, thereby maintaining the mechanical stability of the membrane under high temperature and dry conditions. Murthy et al. [10] and Manahan et al. [11] placed the subgasket between the CCM, GDL, and gasket to allow for better alignment of the exact active area of the CCM and GDL during cell assembly. It has been reported that the misalignment of the electrodes can create measurement errors and deteriorating effects on fuel cell performance. Winkle et al. [12] simulated the measurement error in a single cell with three-electrode configurations and noted that the electrode misalignment led to large errors in overpotential measurements. Chan et al. [13] also found that significant errors in overpotential measurement arose from the dimensional difference between the anode and cathode. When thin electrolyte membranes are used for PEMFCs and DMFCs, the effects of the electrode alignment are more significant on the measurement accuracy [14]. The overpotential is underestimated for the oversized electrode while is overestimated at the opposite electrode. The error becomes large with increasing current density and reaches as high as nearly 10% in a DMFC system. Alder et al. [15] noted that the oversized cathode resulted in a high potential, which induced local oxygen evolution at the seal/electrolyte interface and thus caused failure of sealing. Sompalli et al. et al. [16] reported that the oversized cathode accelerated the membrane failure at the MEA edge even under fully humidified conditions in a short period of time. Therefore, the electrode alignment and subgasket are important in the measurement accuracy and durability of MEAs. In practice, electrode misalignment could often occur when a laboratory MEA is artificially integrated with the subgaskets. The electrode could oversize the defined window due to manufacturing tolerance and the rest of electrode could be embedded in the peripheral region of the subgaskets. So far, asymmetric MEAs (i.e. oversized cathode or oversized anode) have been often adopted to investigate the misalignment effects in the above-mentioned work. However, for symmetric MEAs, the influence of oversized electrodes on cell performances is seldom reported when the MEAs are combined with subgaskets. We present here three subgasketed CCMs shown in Fig. 2, specially designed with the electrolyte membrane, catalyst layer, and carbon layer in the peripheral region. The specifications of these CCMs are listed in Table 1. The peripheral region effects on active area, hydrogen crossover, shorting resistance, proton conduction, and oxygen reduction reaction (ORR) kinetics are examined. Also, their influences on single

Fig. 2 e Schematic diagrams of the three CCMs with subgaskets, showing the surface and cross-section. cell performances are evaluated at different relative humidities under atmospheric air condition.

2.

Experimental

2.1.

Preparation of catalyst-coated membrane (CCM)

The catalyst inks were prepared with Pt/C catalyst (60 wt. %, Johnson Matthey) and Nafion ionomer (5 wt. % solution, EW1000, DuPont). The weight ratio of the solid ionomer to

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Table 1 e Specifications of active region and peripheral region for the three subgasketed CCMs. CCMs Configuration 1 Central region (Active region) Peripheral region (Non-active region)

Area Composition Area Composition

carbon was fixed at 0.8. The mixture was sonicated (Ultrasonic bath, Branson 1510) for 60 min for homogenizing at room temperature. The ink containing carbon black (XC-72, Cabot) and Nafion solution was also prepared in this manner. The CCM was obtained by directly depositing the catalyst ink on the Nafion112 membrane (4 cm  4 cm) on a heated vacuum table according to the diagrams in Fig. 2. For Configuration 3, the catalyst ink was first sprayed onto the central region while the peripheral region was masked by a PTFE film, then the carbon ink was applied to the peripheral region by covering the central region (the catalyst layer area) to obtain the carbon layer with a carbon loading of 0.26 mg cm2 on each side. The Pt loading in the catalyst layers was 0.4 mg cm2 both for the anode and the cathode. The adhesive kapton subgaskets were cut into a hollow square and were bonded to both sides of the CCMs by hot-pressing at 80  C with a pressure of 20 kg/ cm2 for 2.0 min, leaving an active area of 5 cm2 (ca. 2.23  2.23 cm) in the center. Accordingly, the non-active area is 11 cm2 in the peripheral region of the subgasket.

2.2.

Single cell testing

The subgaketed CCM was assembled into a 5 cm2 single cell hardware with straight flow channels (Electrochemical Inc). Carbon paper containing microporous layer (GDL 25 BC, SGL carbon group) was used as the gas diffusion layers (GDLs) to form MEAs without hot-pressing. Electrical heaters and a thermocouple were embedded into the end plates and connected to a fuel cell test station (Scribner, 850 C) to control cell temperature, gas flow rate, and relative humidity. The humidified hydrogen and oxygen (or air) were introduced to the anode and cathode chamber with a flow rate of 200 mL min1 at the anode and 500 mL min1 at the cathode, respectively. The MEA was activated at a constant current to maximize its performance until the cell voltage stabilized. The cell resistance was measured by the current-interrupt technique built in the fuel cell test station. The polarization curves were galvanostatically recorded by fuel cell software (Scribner Associates).

2.3.

Electrochemical measurements

All the electrochemical measurements were carried out at the cell temperature of 70  C. Cyclic voltammetry (CV) was performed on a potentiostat (Solartron 1287A, Solartron Analytical). The anode and the cathode were purged with, respectively, 100% humidified H2 and N2 at a flow rate of 200 mL min1. The anode served as both reference hydrogen electrode and counter electrode (Dynamic hydrogen electrode, DHE). The working electrode (cathode) was scanned in the potential range from 0.04 to 0.9 V vs. DHE at the scan rate of

2

5 cm Pt/C þ Nafion 11 cm2

Configuration 2 2

5 cm Pt/C þ Nafion 11 cm2 Pt/C þ Nafion

Configuration 3 5 cm2 Pt/C þ Nafion 11 cm2 C þ Nafion

20 mV s1. The electrochemical redox activity of hydrogen on the catalyst layer was thereby monitored and curves were recorded by CorrWare software (Scribner Associates). The electrochemical active surface area (ESA, m2/g) was calculated by integrating the voltammetric peaks of hydrogen desorption/adsorption in the potential range of 0.08e0.4 V based on equation (1) as Q  H Ptloading  210mC=cm2

ESA ¼ 

(1)

where [Ptloading] is the Pt loading (g m2) in the cathode, QH (mC) is the charge for hydrogen desorption/adsorption, and 210 mC cm2 is the conversion factor for charge to area on smooth Pt. The double layer capacitance (Cdl, F cm2) can be obtained from the capacitive charge current by equation (2) as Cdl ¼

jdl n

(2)

where jdl (mA cm2)is the capacitance charging current and v (mV s1) is the scan rate for CV experiments. The H2 cross-over current density (ix-over) was measured by potential step voltammetry as reported elsewhere [17] under the same conditions as CV measurements. The working electrode (i.e. cathode) potential was stepped from 0.2 to 0.5 V in 0.1 V increments and 3 min equilibration time. H2 crossover current density (ix-over) and the shorting resistance were obtained from the currentepotential plot. To calculate the proton resistance of the catalyst layer, electrochemical impedance spectroscopy (EIS) was conducted on a frequency response analyzer (FRA, Solartron 1260, Solartron Analytical) combined with a potentiostat (Solartron 1287A, Solartron Analytical) and ZPlot software (Scribner Associates). The amplitude of sinusoidal signal was set at 10 mV rms over the frequency range of 4 KHz to 0.1 Hz. A DC potential of 0.45 V vs. DHE was applied to the cathode under the same conditions as the CV experiments.

3.

Results and discussion

3.1.

Electrochemical surface area (ESA)

Fig. 3 shows the CV curves for the CCMs with Configurations 1, 2 and 3 at 100% relative humidity (RH). The curves exhibit the typical peaks for Pt/C electrode. The potential range of 0.04e0.9 V was chosen to avoid the H2 evolution at low potentials and platinum oxidation at high potentials. According to equations (1) and (2), the calculated ESAs and double layer capacitances (Cdl) are listed in Table 2. ESA reflects the number

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and 0.40 V as Configurations 1 and 2. The calculated ESA of 30.12 cm2 is listed in Table 2, which is lower than that of Configuration 2, but very close to that of Configuration 1. It can be inferred that the increase in ESA for Configuration 2 is attributed to the catalyst layer in the peripheral region. The double layer capacitances (Cdl) for the MEAs are 11.69, 22.20, and 17.79 mF cm2, respectively, for Configurations 1, 2 and 3, which is in accordance with the ESA values, as shown in Table 2. Compared to Configuration 1, the larger capacitance in Configuration 3 might arise from the carbon in the peripheral region.

35 Configuration 2 Configuration 3

15 5 -5 -15 -25 -35 -45 0

0.1

0.2

0.3

0.4 0.5 0.6 0.7 Potential (V vs. DHE)

0.8

0.9

1

3.2.

Fig. 3 e Typical cyclic voltammograms of the cathode catalyst layers in three CCMs at 70  C and 100% RH.

of Pt sites that are in contact with the electrolyte and available for electrochemical reactions in the electrode, and is commonly used to evaluate the activity of catalysts and catalyst layer. The values of ESA are 29.7 and 36.4 m2 g1, respectively, for CCMs with Configurations 1 and 2. The ESA of Configuration 2 was much higher than that of Configuration 1. It should be noted that these values were based on the electrode area of 5 cm2. Under the same conditions, the ESAs of the two CCMs should be equal or close as they are identical electrodes except for the peripheral region. Considering the discrepancy in configurations, the catalyst layer underneath the subgasket in Configuration 2 is the only contributor to the larger ESA. Assuming the ESA of Configuration 2 is equal to that of Configuration 1, the geometrical area of active region for Configuration 2 should be 6.14 cm2 according to equation (1), which is larger than that of the defined window (5 cm2) in the subgasket. It indicates that the active area was extended to the peripheral region in Configuration 2. However, there is only a very small part of the catalyst layer involved in hydrogen desorption/adsorption reaction compared to 11 cm2 peripheral region. Most of the catalyst layer still remained dormant under the subgaskets. Quantitatively, an extension distance of 0.12 cm starting from each inner edge of the defined window was activated underneath the subgaskets. To verify this hypothesis, the catalyst layer in the peripheral region of Configuration 2 was replaced with the carbon layer comprising of carbon powder and Nafion ionomer. This CCM is denoted as Configuration 3. It can be observed from Fig. 3 that the CV curve of Configuration 3 exhibits the same features of hydrogen adsorption/desorption peak between 0.04

Table 2 e Electrochemical surface area (ESA), double layer capacitance (Cdl), H2 cross-over current density, and shorting resistance measured at 100% RH for the three subgasketed CCMs. Cdl ESA (m2 g1) (mF cm2) Configuration 1 Configuration 2 Configuration 3

29.68 36.44 30.12

11.69 22.20 17.79

Shorting Hx-over resistance current (mA cm2) (Ohm cm2) 1.162 1.244 1.957

757.2 392.7 569.5

H2 cross-over and shorting resistance

Potential step voltammetry was employed to determine the H2 oxidation current at the cathode, ix-over, which reflects the permeation rate of H2 through the CCM. Crossed-over H2 was oxidized by applying a potential to the cathode and the resulting current was measured. Fig. 4 shows the hydrogen crossover current densities measured in the potential range of 0.2e0.5 V over a period of 3 min at each potential to obtain steady-state values. The limiting current densities against the applied potentials on the cathode are depicted in Fig. 5. It can be observed that the current density increases linearly with the potential, which is an indication of an electrical shorting in CCM and is manifested in a positive slope in the currentepotential plot. The electrical shorting current increases linearly with the applied potential and is inversely proportional to the shorting resistance. If there is no significant electrical shorting in CCM, the crossover current should be equal to the measured current and reaches a plateau at high potentials. After a linear fitting, the interpolation of the data to the y-axis represents ix-over, and the inverse slope represents the shorting resistance of CCM as described elsewhere [17]. The fitting results are also listed in Table 2. The values of ix-over are 1.162, 1.244 and 1.957 mA cm2, respectively, for Configurations 1, 2, and 3. Assuming that 1.162 mA cm2 is the oxidation current density of H2 permeating through the electrolyte membrane in the defined window, H2 crossover current densities are 0.1378 and 0.795 mA cm2 through the peripheral region, respectively, for Configurations 2 and 3. Potential “tents” are built at the edges of the catalyst layers and subgaskets when the electrodes oversize the defined 6 Configuration 1

Current density (mA cm -2)

-2

Current density (mA cm )

Configuration 1

25

5

Configuration 2 Configuration 3

4 3 2 1 0 0

100

200

300

400 500 Time (s)

600

700

800

Fig. 4 e Potential step curves for measuring the hydrogen crossover of the three CCMs at 70  C and 100% RH.

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3 Current density (mA cm -2)

Configuration 1 Configuration 2 Configuration 3

2.5

2

1.5

1 0.1

0.2

0.3 0.4 Potential (V)

0.5

0.6

Fig. 5 e H2 crossover current density versus potential plots for the three CCMs.

window in the subgasket [18]. H2 easily goes through these “tents” and diffuses to the cathode besides H2 permeation through the electrolyte membrane. Compared with the catalyst layer, higher porosity of carbon layer further facilitates the diffusion of H2. The H2 permeation does not have big impact on the cell output when the ix-over accounts for small percentage of the produced current density (<3% when operating at 60 mA cm2, and <0.2% when operating at 1.0 A cm2) for an operating single cell [17]. However, direct access of O2 and H2 could facilitate the formation of peroxide and hydroperoxide radical species, thus leading to accelerated chemical degradation of the membrane [19]. On the other hand, the shorting resistances are 757.2, 392.7, and 569.5 Ohm cm2, respectively, for Configuration 1, 2, and 3. The small shorting resistance can aggravate direct conduction of electrons between the electrodes, which is a source of power loss within a fuel cell. Overall, the catalyst layer or carbon layer underneath the subgaskets exerts negative effect on the cell performance.

RH. Fig. 6 presents the impedance spectra of the catalyst layers in the three CCMs. They are near 45-degree linear curves due to the protonic resistance in the catalyst layer and change gradually to 90-degree with decreasing frequency [17,20]. A transmission line equivalent circuit in Fig. 7a was built to fit the impedance data in Fig. 6. A distribution element, DX6 (open circuit terminus) represents the transmission line model consisting of a series of differential elements such as proton resistance, double layer capacitance, and charge transfer resistance, shown in Fig. 7b. R1 denotes the ohmic resistance and L1 is the high frequency inductance. The obtained proton resistance and membrane resistance are listed in Table 3. As can be seen, the three CCMs possess similar values, indicating that the configurations of the CCMs do not have much effect on proton resistance and membrane resistance compared to their impacts on ESA and H2 crossover current. The conductivity of Nafion ionomer and membrane is merely related to the hydration level in the fuel cell.

3.4.

In the kinetic region (E > 0.8 V), the Tafel plots for ORR were obtained by plotting IR-corrected electrode potentials against the logarithm of current density. Fig. 8 shows the Tafel plots of ORR on the cathodes for three CCMs. It is noted from Fig. 8 that the effects of CCM configurations on ORR kinetics are not very pronounced. The Tafel slopes are very close for the three CCMs, possessing slope of 68.8 mV dec1, which is typical value for ORR at Pt/C electrode. In this regard, the configuration of the CCM did not exert any influence on the ORR kinetics. It should be pointed out that it is difficult to determine accurately the Tafel slope in MEAs by using some empirical equations [21]. Tafel slope is also influenced by other factors such as reactant humidity, ionic resistance, electronic resistance, reaction mechanism, and anode polarization [22].

3.5. 3.3.

Proton resistance

Proton resistance in the catalyst layer was evaluated by in-situ EIS measurements on single cells operated with H2/N2 at 100%

ORR kinetics

Polarization curves under different RHs

Fig. 9a shows the cell performance of three CCMs operated at 100% RH and ambient pressure with H2/air. The high frequency resistances (HFR) are also shown in Fig. 9a. The HFR

-2 Configuration 1 Configuration 2 Configuration 3

-0.4 -0.3

-1

Z'' (Ohm.cm )

Z'' (Ohm.cm2)

-1.5

-0.5

-0.2 -0.1 0 0.1 0.2 0.3 0

0.1

0.2 0.3 Z' (Ohm.cm )

0.4

0.5

0

0

0.5

1 Z' (Ohm.cm2)

1.5

Fig. 6 e Impedance spectra for measuring the proton resistance and membrane resistance of the three MEAs at 70  C and 100% RH.

2

Fig. 7 e (a) Equivalent circuit for analyzing the proton resistance in the cathode catalyst layer and (b) transmission line model of the catalyst layer showing proton resistance (Rp), charge transfer resistance (Rct), electronic resistance (Re), and double layer capacitance (Cdl).

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1

Configuration Configuration Configuration 1 2 3

0.8

94.9

148.4

91.6

135.5

91.0

Configuration 2 Configuration 3 HFR-Configuration 1

Volatage (V)

0.6

200

0.4

137.1

100

0.2

0 0

600

800

1000

0 1200

1 Configuration 1

(b)

Configuration 3

0.8

Configuration 2'

0.6 0.4 0.2 0 0

200

400

600

800

Current density (mA cm-2)

1000

1200

Fig. 9 e H2/air polarization curves of the three CCMs at 70  C and 100% RH (anode/cathode).

rate of peroxide and hydroperoxide radical species at elevated temperatures and low RH, consequently leading to accelerated local chemical degradation of the membrane and ionomer [19], but it might not contribute to the lower performance of Configurations 2 and 3 in a short-term operation. On the other hand, the carbon layer and the catalyst layer in the peripheral region could absorb part of the water from the active catalyst layer, which aggravates the dehydration of the active catalyst layer under dry condition and results in high cell resistance and consequent low performance. In our work, the structure and composition of Configuration 3 is very similar to that of the water transfer

1

Configuration 1

800 Configuration 1

Configuration 2

0.88

400

Current density (mA cm )

0.9 0.89

200

-2

Volatage (V)

corresponds to the cell resistance and its predominant part is membrane resistance. Generally, HFR decreases with increasing current density that produces more water and improves the hydration level of MEAs, thus leading to a decrease in cell resistance. The performances of the three CCMs have small differences in the activation and ohmic region, especially for Configurations 2 and 3, which is attributed to the extended reaction area and decreased areal resistance in Configuration 2. In the limiting current region, the performances are almost close for these MEAs. When the produced current of Configuration 2 was normalized by the real active area of 6.14 cm2, the corresponding curve is denoted as Configuration 20 in Fig. 9b. Interestingly, it is found that the polarization curve of Configuration 20 overlaps well with those of Configurations 1 and 3 in the low current density region. However, the discrepancy is very significant in the limiting current density region, which suggests that the CCM configurations have significant impact on the cell performance in the low current density region without influence on the limiting current. When the RH of the reactant was further reduced to 30%, Configuration 1 CCM shows the best performance compared to the other two CCMs, while Configurations 2 and 3 CCM exhibit similar performances in the whole current range as shown in Fig. 10. Regarding Configurations 2 and 3, the space of “tents” formed in the peripheral region becomes large under dry condition due to the shrinkage of the catalyst and carbon layers, resulting in a diffusion of more hydrogen and air to the opposite sides. It was reported that direct access of O2 or H2 gas through the “tent” could enhance the formation

Configuration 2

0.8

Configuration 3

Configuration 3

0.87

600

HFR-Configuration 1

0.86

Voltage (V)

IR-corrected cell voltage (V)

300

HFR-Configuration 2 HFR-Configuration 3

0.85 0.84

HFR-Configuration 2

0.6

HFR-Configuration 3

0.83

200

0.2

0.82

400

0.4

0.81 0

0.8 10

Current density Log (i+ix-over)

10 0

Fig. 8 e Tafel plots of H2/O2 polarization curves of the three CCMs at 70  C and 100% RH (anode/cathode).

0

200

400 600 800 1000 Current density (mA cm-2)

1200

0 1400

Fig. 10 e H2/air polarization curves of the three CCMs at 70  C and 30% RH (anode/cathode).

HFR (mOhm.cm2)

Membrane resistance (mOhm cm2) Proton resistance (mOhm cm2)

400

Configuration 1

(a)

HFR (mOhm.cm2)

Table 3 e Proton resistance and membrane resistance for three subgasketed CCMs measured at 100% RH.

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region (WTR) proposed by Wang et al. [23]. They reported that the carbon layer surrounding the catalyst layer humidified the catalyst layer and improved the cell performance of MEA at reduced RH, but the area ratio of WTR to MEA was carefully controlled between 28 and 60% and the best cell performance was achieved at 28%. However, the non-active area (carbon layer) is almost two times larger than the active area in Configuration 3. Water could not easily and timely diffuse back to the active layer due to the higher absorbing capacity of the non-active area. This is the reason that the carbon layer did not work effectively like WTR to enhance cell performance. It should be noted that the effects are investigated here with a laboratory-scale small single cell where the ratio of the peripheral region to the active area is very large, while this ratio will be much smaller in the large-scale and commercial cell. Therefore, the effects of non-active area under subgasket could be different from what has been found in this work. Further work could focus on assessing the effects in large cells and shed more light on the relevance of this study for practical cells.

4.

Conclusions

Three subgasketed CCMs with different configurations were designed and their effects on ESA, H2 crossover, shorting resistance, proton resistance, and ORR kinetics were also examined. Under fully humidified conditions, the activation of a small part of the catalyst layer underneath the subgaskets led to higher ESA, higher H2 crossover, and smaller shorting resistance. The configurations of the subgasketd CCMs did not have impact on the proton resistance in the catalyst layer, ORR kinetics, and cell performance in the limiting current region. However, it exerted adverse effects on cell performance in the low current region under dry conditions.

Acknowledgment This work was supported by the Office of Naval Research MURI grant No. N00014-07-1-0758.

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